Electronic information devices, particularly various types of mobile electronic information equipment, have become highly popular. Most of those types of electronic information equipment use batteries and contain a power converter, such as a DC-DC converter. The power converter is generally constructed as a hybrid power supply module in which discrete parts of active components, such as switching elements, rectifiers, and control ICs, and passive components, such as magnetic parts, capacitors, and resistors, are packaged on a ceramic board or a plastic printed circuit board.
With a requirement for reduction in size, thickness, and weight of various types of electronic information equipment, including the mobile devices, the built-in power converters need to be smaller and lighter. The miniaturization of the hybrid power module has been developed by an MCM (multi chip module) technique and a technique of laminated ceramic parts. Since discrete parts are packaged arranging on the same substrate, however, the reduction in packaging area of the power module is restricted. Particularly, a magnetic part, such as an inductor or a transformer, takes up a significant amount of volume in comparison with an integrated circuit. Accordingly, the magnetic part plays the most critical role in the final size and thickness of the electronic equipment.
To reduce the size and thickness of a magnetic component, two approaches are conceivable. One approach is to make the magnetic component as small and thin as possible, and package it on a planar board. Another approach is to make the magnetic component with a thin film and mount it on a silicon substrate. An example has been reported recently in which a thin micro magnetic element (coil, transformer) is mounted on a semiconductor substrate by applying a semiconductor technology. In particular, Japanese Unexamined Patent Application Publication No. 2001-196542, discloses a planar magnetic component (a thin inductor, a thin film magnetic induction device) formed using a thin film technology, where a thin film coil is sandwiched between a magnetic substrate and a ferrite substrate, and formed on a surface of a semiconductor substrate where semiconductor parts, such as a switching element and a control circuit, are formed. By such structure, it has become possible to reduce the thickness and the packaging area of the magnetic element. Forming such structure, however, requires a vacuum process, thus raising the manufacturing cost. In addition, it needs to laminate a multiple of magnetic films and insulator films for use at high electric current, which increases the costs very much.
Japanese Unexamined Patent Application Publication No. 2002-233140 (FIG. 1) discloses a type of a planar magnetic element, where a resin mixed with magnetic fine particles is filled in a gap of a spiral coil conductor, and sandwiched between ferrite substrates. In this approach, however, since the inductance of the coil conductor is approximately proportional to the number of turns of the spiral, it is necessary to increase the number of turns to achieve a large inductance. When the number of turns is increased without increasing the packaging area, the cross-sectional area of the coil conductor must be decreased. That is, to obtain a large inductance, it is necessary to decrease the cross-section of the coil conductor and to elongate the conductor wire. The decreased cross-sectional area of the coil conductor and the elongated conductor wire bring about an increase in the DC resistance of the coil conductor and an increase in power loss.
To solve this problem, Japanese Unexamined Patent Application Publication No. 2004-274004 (corresponds U.S. Pat. No. 6,930,584 B2, the disclosure of which is incorporated herein by reference, and Chinese Patent Application Publication No. CN 1525631 A1) discloses a thin magnetic element that comprises a magnetic insulating substrate, and a solenoid coil in which a first coil conductor formed on a first principal plane of the magnetic insulating substrate, a second coil conductor formed on a second principal plane of the magnetic insulating substrate, and a connection conductor formed in a through-hole passing through the magnetic insulating substrate are connected. Such structure provides a microminiature, thin power converter in which semiconductor elements and terminals for connection to a circuit board can be formed at the same time in the process for forming the through-holes and the coil conductor on the magnetic insulating substrate, and the IC chip only needs to be mounted on the magnetic insulating substrate for forming the solenoid coil, eliminating necessity for a separate packaging substrate.
Such microminiature power converter incorporates the through-hole formed in the magnetic insulating substrate, the coil conductors electrically connected through the through-hole and formed on the first principal plane and the second principal plane, and further an electrode (connection terminal) formed on the first principal plane for electrical connection to the semiconductor element, and another electrode (packaging terminal) formed on the second principal plane for electrical connection to a printed circuit board that is used in actual operation. The proposed constitution provides a power converter that achieves miniaturization and reduction in thickness while limiting the number of parts composing the converter to a minimum.
It has been reported in Japanese Unexamined Patent Application Publication No. 2006-280127 (corresponds to United States Patent Application Publication No. 2006/227518 A1 and Chinese Patent Application Publication No. CN 1841901 A1) that a malfunction hardly occurs when the terminals (electrodes) formed in the peripheral region of the ferrite substrate are arranged along the direction of Y-axis perpendicular to the axial direction (X-axis direction) of the solenoid coil, the magnetic flux density being low in the peripheral region along the Y-direction.
The structures disclosed in the above identified references have, as shown in FIGS. 11A and 11B, bonding electrodes (electrode 82 on the second principal plane and electrode 88 on the first principal plane) in the peripheral region of the thin film magnetic induction device for bonding with a semiconductor element (IC chip 80). Since the coil of the thin film magnetic induction device having a configuration of solenoid coil must be located in the central region of the ferrite substrate 86, the bonding electrodes (electrodes 82, 88) for bonding with the IC chip 80 are necessarily located in the peripheral region of the ferrite substrate 86. When the electrodes are arranged in the side region in the direction parallel to the coil width direction as in the case of FIGS. 11A and 11B where the electrodes 82, 88 are arranged along the four peripheral sides, the length and the number of turns of the coil are restricted by electrodes A arranged in the direction parallel to the coil width direction.
Among the characteristics of a thin film magnetic induction device, an inductance value largely depends on the length in the direction of the coil axis (size of the element). The size of the element primarily affects performance. The size of the semiconductor element (IC chip 80), on the other hand, can be restricted by the performance. There is no intrinsic correlation between the size of the semiconductor element and the size of a thin film magnetic induction device. Nevertheless, since the semiconductor element is mounted on the magnetic induction device and electrodes are arranged in the peripheral region, the size of the semiconductor element is eventually restricted by a size of the magnetic induction device.
Even though the semiconductor element can be minimized, if a thin film magnetic induction device is not minimized, the semiconductor element must have a large size, inhibiting reduction in the overall size. When the thin film magnetic induction device is minimized in correspondence with the reduced size of the semiconductor element, the performance of the magnetic induction device becomes worse, resulting in degraded efficiency of the power converter. Moreover, a large induced voltage is generated on the electrodes (a voltage generated by an action of magnetic field of the coil on the connection conductor of the electrodes), which is apt to bring about malfunctions of the semiconductor element (an IC).
In FIGS. 11A and 11B, reference 81 designates a stud bump connecting the IC chip 80 and the electrode 88, references 83 and 85 designate through holes accommodating connection conductors that connect front and back electrodes and front and back coil conductors, reference 87 designates a protective film for protecting the coil; and numeral 89 designates an under fill resin.
There remains a need for a microminiature power converter with its thin film magnetic induction device having a higher inductance value, while reducing induced voltage appearing at its electrodes. The present invention addresses this need.